1. Field of the Invention
This invention relates to improvements in automatic wire bonders, and more particularly to improvements in wire bonders of the type in which a plurality of wires are connected between a first set of predetermined points of possible varying elevations to a second set of predetermined points on an underlying substrate.
2. Description of the Prior Art
In the field of wire bonders, for application, for example, in attaching non-insulated, conducting leads, or wires, between an integrated circuit chip, or component such as a transistor, resistor, capacitor or the like, to an underlying substrate, several considerations are of concern.
One consideration of acute interest is that often times the elevation above the substrate to which connection is to be made is not constant. For example, in making wire connections between an integrated circuit chip on which connection pads are provided onto an underlying substrate, the levelness or flatness of the chip on the substrate may vary due to variations in chip mounting techniques or height differences upon the chip itself. The resulting height variations may be from pad to pad on the same chip, or the height variations may be elevation variations from chip to chip on the same substrate or even on a different substrate.
Ideally, it is desired that the wire connection achieved be made by a wire of such configuration, or dress, that it does not extend so high above the substrate to which connection is made as to short against a cover or package into which the circuit is intended to be mounted. On the other hand, if the wire is made too short, it may short against the edge of the chip, also causing undesirable consequences.
It has been found that a particularly suitable shape for the wire is in the form of a portion of a circle immediately adjacent the chip and a straight line portion extending from the circular portion to the substrate onto which the connection is to be made. The connection to the circuit is usually by a "ball" bond, and the connection to the underlying substrate is usually by a "wedge" bond, such ball and wedge bonds being well known in the art.
It has been found that a desired wire dress can be approximately achieved manually by feeding the wire from a spool through a capillary tube and manually tracing a pattern by the capillary tube to force the wire to the desired dress pattern. The manual performance of this procedure is difficult to learn by machine operators, as it must be done by "cut and try" methods, and is not conducive to repeated accurate wire bonding.
More specifically, the manual method previously developed by applicant is to attach the wire to the chip by a ball bond, then feed out an additional portion of wire from the capillary tube. The capillary tube is then moved in a direction away from the direction the wire is to ultimately run, and a downward pressure is applied to the wire, causing it to bend to assume an arcuate or circular shape. An additional length, sufficient to reach the connection point of underlying substrate is fed out (approximately) through the capillary tube, and the tube is moved to the connection point and the wedge bond is made. The capillary tube is then raised, allowing a small measured portion of wire to be pulled from the capillary. When the desired amount of wire is pulled from the capillary, the wire is clamped so that no more wire will leave the capillary. An additional vertical pressure is applied to the capillary, causing the wire to break at the wedge bond, resulting in a pig-tail extending from the capillary tube. A high voltage lead is then moved into contact with the pig-tail, melting it into the form of a ball. The ball is then brought into position at a new location on the component substrate, and the process is repeated.
In the past, in the field of automatic wire bonders, the capillary was first moved downwardly until the ball was brought into contact with the pad to which connection is intended to be made. After the bond has been made, the capillary was moved directly up and then to a location above where the wedge bond was to be made. Finally the capillary was lowered to make the wedge bond. The wire naturally bent at a location along its length immediately next to the first made ball bond because of the moment exerted onto it by the capillary movement and because this area is weakest due to the heat applied during the formation of the ball.
The resultant wire configuration, or dress, was usually a fairly straight line with a slight curvature immediately adjacent the ball bond. This configuration resulted in several problems, including, often times, the shorting of the wire against the chip on which it was made. Another problem encountered, especially where a fairly long length of wire is necessary for connection, is that the wire sometimes sags or deforms, shorting against other wires or other parts of the substrate on which the wedge bond is to be made. Another problem encountered by prior art automatic bonders is that the wire is sometimes streched or deformed, sometimes even broken, when it is moved to the location for connection. These problems become especially acute if the chip from which connection was to be made has not been mounted with precision. That is if, for example, an integrated circuit chip were to be mounted at an angle from that position for which the bonder was programmed, some of the wires would be too long and some too short.
Because of the inability of prior automatic bonders to achieve a consistent, well formed wire dress, the use of automatic bonders has been limited to use in less dense configurations in which the wire dress is not critical. In some prior art applications, in efforts to minimize the effects of these problems, especially the sagging of wires, the entire substrate has been coated with an insulating material, except at the points at which connections are desired. This results in additional expense and extra steps in the manufacturing process. In the more dense connection applications, those configurations have been necessarily fabricated manually, but, because of the large number of connections and the difficulty of achieving consistent wire dress, even manually, yields ordinarily achieved have not been particularly high.
In the past, when an unacceptable bond has been made to a chip manual efforts were required to correct the bond, such as attempting to raise a sagging connection. However, one solution which was not available was to bond a new wire in its place, since manufacturers frequently refuse to accept circuits containing ball bond to ball bond connections. Thus, if an unacceptable, uncorrectable connection is made, the entire chip must be chiseled from the substrate and a new one mounted in its place. Then all of the connections must be reestablished. Presently, many chips used in hybrid circuits of the type described herein are quite expensive; consequently, an additional pressure is found to ensure that the wire connections are properly achieved the first time.